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Atoms and Molecules

Gujarat Board · Class 9 · Science

Quick revision notes for Atoms and Molecules — Gujarat Board Class 9 Science. Key concepts, formulas, and definitions for last-minute revision.

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Key Topics to Revise

1

Laws of Chemical Combination

  • Two important laws of chemical combination were established by Lavoisier and Proust through careful experimentation.
  • Law of Conservation of Mass: In any chemical reaction, the total mass of reactants equals the total mass of products. Mass is neither created nor destroyed.
  • Example of Conservation of Mass: In a reaction where 5.3 g sodium carbonate reacts with 6 g acetic acid, the products (2.2 g CO₂ + 0.9 g water + 8.2 g sodium acetate) also add up to 11.3 g, which equa
2

Dalton's Atomic Theory

  • John Dalton (1766–1844) was a British chemist who gave the first scientific atomic theory in 1808.
  • Dalton picked up the philosophical idea of atoms (from Greek philosopher Democritus) and gave it a scientific basis using the Laws of Chemical Combination.
  • The 6 postulates of Dalton's Atomic Theory are: (1) All matter is made of tiny particles called atoms. (2) Atoms are indivisible and cannot be created or destroyed in chemical reactions. (3) Atoms of
3

Atoms – Size, Symbols and Atomic Mass

  • Atoms are extremely tiny — atomic radius is measured in nanometres (1 nm = 10⁻⁹ m). A hydrogen atom has a radius of about 10⁻¹⁰ m.
  • More than millions of atoms stacked together would make a layer barely as thick as a sheet of paper.
  • Symbols of Elements: Dalton was the first to use symbols. Berzilius suggested one or two letter symbols. Today IUPAC approves all names and symbols.
4

Molecules – Types and Atomicity

  • A molecule is the smallest particle of an element or compound that can exist independently under ordinary conditions and shows all the properties of that substance.
  • Molecules are formed by the chemical bonding of two or more atoms (same or different elements).
  • Atomicity: The number of atoms present in one molecule of an element or compound.

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Full Notes

Key Concepts

This fundamental law states that massIn any pure chemical compoundJohn Dalton proposed that all matterAtoms are the smallest particlesAtomic mass is the relative mass

Frequently Asked Questions

What are the important topics in Atoms and Molecules for Gujarat Board Class 9 Science?
Key topics in Atoms and Molecules include Atoms and Molecules – Complete Chapter Overview, Atoms and Molecules - Complete Concept Map, Atoms and Molecules – Complete Chapter Overview. These are the concepts Gujarat Board Class 9 examiners draw on most — study them first, then practise related questions.
How to score full marks in Atoms and Molecules — Gujarat Board Class 9 Science?
Understand the core concepts first, then work through the 59 practice questions available for this chapter. Revise formulas and definitions regularly, and use flashcards for quick recall before the exam.

Sources & Official References

Content is aligned to the official syllabus. Refer to the board website for the latest curriculum.

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